1. Field of the Invention
The present invention relates to flexible hard pipes which are pipes to be used mainly for transporting air and water, for protecting cables, or the like, and which are pressure-containing pipes adapted to be used on or above the ground, or buried under ground.
The present invention particularly relates to hard pipes which, in spite of their pressure-containing property, are flexible so as to facilitate pipe laying and the transportation of pipes even when the pipes are buried underground and receive externally exerted pressure or are used as pipes carrying high-pressure fluid.
2. Background Art
As conventional hard pipes, there are known not only drawn pipes but also metal pipes which are formed such that a belt-like member is helically wound with its side edges abutted and seam-connected with each other by welding or the like. On the other hand, there are also pipes which are galvanized or the like or which are lined with a synthetic resin material over their inner peripheral surfaces so as to maintain an anti-corrosion property. Further, there have been generally used synthetic resin pipes containing no metal, such as so-called vinyl chloride pipes, which are extruded and molded from only a hard vinyl chloride material, or the like. Further, synthetic resin pipes have been already widely used which are formed such that a belt-like member made of synthetic resin is helically wound and opposite side edges thereof are superimposed and fused with each other.
However, an enormous investment in plant and equipment is required in manufacturing the well known drawn pipes, and much trouble and cost and required in seam-welding the latter seam-connected metal pipes. Significant equipment and cost must be required in galvanizing and in lining with synthetic resin. Particularly, it has been technically difficult to uniformly apply anti-corrosion treatment on the entirety of the inner surface of elongated pipes. Further, the synthetic resin pipes in which pipe bodies are formed by successively winding a synthetic resin belt-like member have superior advantages in that the pipes are light so that transportation is facilitated when compared with the pipes made of metal. Furthermore, anti-corrosion treatment is unnecessary. However, such wound synthetic resin pipes have such disadvantages that yield strength relative to tensile strength in the axial direction of pipe is not always sufficient so that the pipes are stretched in the axial direction of pipe when a high-pressure fluid is passed through the pipes and are apt to lack compressive strength against external pressure, so that the pipes did not always have enough strength for use for transmitting high-pressure fluid or for the purpose of being embedded underground.